Mobile ad-hoc networks are known in which a number of users intercommunicate on a network established when one or more of the users arrive at a scene or given area and the ad-hoc network is set up. These ad-hoc networks have been known to have been used for incident command in emergency management scenarios when a number of mobile units arrive at an incident site at which an ad-hoc network is created. The ad-hoc network is created on the spot when at least one of the mobile units arrives at the incident scene and adds other nodes when additional units arrive within the ad-hoc network coverage area.
From a military point of view, when troops are deployed in a theater and arrive on-scene, it is important to be able to immediately establish an ad-hoc network so that all of the troops can communicate with each other. Unlike the civilian use in which there may be an incident commander, in a military situation it is not desirable to have a single identifiable incident commander. One of the reasons is that by taking out the incident commander, one causes the entire network to fail. Thus, there must be no single central point of failure such as a base station that would create a vulnerability in which the enemy could completely disrupt communications simply by taking out one node.
Moreover, since the military is a highly mobile force, this means that they cannot typically set up fixed infrastructure. Thus, the network must be adaptable enough to handle the node movement in and out of the network coverage area.
While adaptive ad-hoc networks have been utilized in the past, there is a need for multiuser detector capability to handle large numbers of troops. The multiuser detector (MUD) capability permits handling communications on the same frequency and at the same time. Thus, large numbers of units can intercommunicate on a single channel.
It is the main feature of a multiuser detector system that users can communicate on the same frequency at the same time and still have the messages separable so that a message from one user does not interfere with a message from another user.
It will be appreciated that any real implementation of a physical layer for a multiuser detector enabled network will have a finite dynamic range. The dynamic range, aka near/far separation, defines the maximum power spread between multiple users being received without significant error rate. The finite dynamic range of a real system is due to many factors including algorithm implementation in fixed point, analog-to-digital converter quantization, filter effects and channel dynamics such as Doppler and multipath. In general, for ad-hoc systems there is a so-called near/far separation problem in which different signals are coming in with different powers. While in cellular networks having a fixed base station or tower, power control is used to assure that all signals come in at an identical receive power. However, in an ad-hoc system in which there are multiple users there is no ability to vary the power of each of the transceivers such that the receive power at each of the receivers is the same. Thus, it is important to be able to adjust transmit power such that the received signals at any node exist between a maximum receive power and a minimum receive power.
In MUD systems, without power control for instance 90% of the communications nonetheless arrive with acceptable error rates. However, 10% of the time there are significant error rates when the receive power at a node is out of the fixed MUD dynamic range. This means that 10% of the time the system will not work, which is unacceptable. With a multiuser detector, the dynamic range can be expressed as the difference in power between maximum receive power and minimum receive power for all of the nodes.
This finite dynamic range means that there are certain circumstances based on the positions of the users in which the dynamic range is exceeded. There is therefore a need to deal with the finite dynamic range when operating in a mobile ad-hoc network.
While 30 dB is considered to be a relatively large dynamic range, more complex implementations can achieve a 40 or 50 dB dynamic range. However, achieving a greater than 30 dB dynamic range involves systems that are bigger in size, weight and power and result in costly radios.
Power adjustment for the individual transceivers in the mobile ad-hoc network is one way to be able to establish reliable communications between all of the nodes of the network. Most importantly, there must be a minimum signal-to-noise ratio or interference-to-noise ratio that must exist in order that one user at one node can communicate with other nodes. Moreover, the transmission from one node cannot overpower other nodes which would result in interference with the other nodes.
One way of establishing an acceptably low error rate is to make sure that the receive power from each of the transceivers at each of the nodes is such as to provide a received signal with a high signal-to-noise ratio and a high signal to interference plus noise ratio.
It therefore may be desirable to utilize power control in multiuser detector ad-hoc networks, with the goal of a signal arriving at a minimum desired power level or within a power spread. It may also be desirable to use a power related scheduling program to schedule packets at a different time.
While with cellular towers and fixed base stations signals are to arrive at identical powers, when looking at an ad-hoc network for instance having five users trying to send to five different users, the problem of adjusting power is indeed complex. Note that it is actually impossible to have signals from every user arrive at every other user at the same power level due to the different locations of the users. When one adds multiuser detection to the mix it is important that the received signals from all five users are within the finite dynamic range of the system.
By way of further background, Power Control (PC) and Power Aware Scheduling (PAS) are well studied topics in the context of cellular networks as is disclosed in Dahlman, E., Parkvall, S., Skold, J., and Beming, P. 2008 3G Evolution, Second Edition: HSPA and LTE for Mobile Broadband. 2. Academic Press. Cellular networks, which operate in hub-and-spoke topologies, use Power Control to minimize the interference from other handsets, with the goal controlling all handset's power to within a couple dB at the base station as is disclosed in Varrall, G. and Belcher, R. 2003 3G Handset and Network Design. John Wiley & Sons, Inc. The Power Control requirement is necessary because cellular networks currently use single user receivers which treat interference from other users as noise. Power Control in cellular networks has the added benefit of reducing the overall transmit power level of the handsets, thus saving battery life. Power Aware Scheduling, where the base station makes scheduling decisions based off of received power levels from all handsets, is used in cellular networks to schedule the handset with the best instantaneous channel.
More recently, PC and PAS for Multiple Access Networks (MANETs) has become an increasingly studied topic. PC and PAS in MANETs, similar to cellular networks, have been shown to increase network capacity and maximize battery life as is disclosed in R. Ramanathan and R. Rosales-Hain, “Topology Control of Multihop Wireless Networks Using Transmit Power Adjustment,” Proc. IEEE INFOCOM, pp. 404-413, 2000; and Singh, S., Woo, M., and Raghavendra, C. S. 1998. Power-aware routing in mobile ad-hoc networks. In Proceedings of the 4th Annual ACM/IEEE international Conference on Mobile Computing and Networking (Dallas, Tex., United States, Oct. 25-30, 1998). W. P. Osborne and D. Moghe, Eds. MobiCom '98. ACM, New York, N.Y., 181-190. DOI=http://doi.acm.org/10.1145/288235.288286
Although MUD based receivers theoretically require no power control, in practice they can only support separate users with a limited power spread, 30 dB in the case of the DARPA Interference Multiple Access (DIMA) system.
Other related references include R. Learned et al. “Interference Multiple Access Wireless Network Demonstration Enabled by Real-Time Multiuser Detection”, Proc IEEE RWS, Orlando, Fla., 2008; and Y. Eisenberg, K. Conner, M. Sherman, J. Niedzwiecki, R. Brothers, “MUD Enabled Media Access Control for High Capacity, Low-Latency Spread Spectrum Comms”, Proceedings of the IEEE MILCOM, Orlando, Fla., 2007.